Electrical voltage transformation apparatus



E. D. LILJA March 19, 1940.

ELECTRICAL VOLTAGE TRANSFORMATION APPARATUS Filed March 23, 1936 3Sheets-Sheet l INVENTOR Edgar .17. Lil a BY m f V ATTORNEY E. D. LILJAMarch 19, 1940.

ELECTRICAL VOLTAGE TRANSFORMATION APPARATUS Filed March 2:5, 193s 3Sheets-Sheet 2 V N .r 4......

E. D. LILJA March 19, 1940.

ELECTRICAL VOLTAGE TRANSFORMATION APPARATUS Filed March 23, 1936 3Sheets-Sheet 5 T '29 7. L/u: Var/m5 EXC/TAT/ON C URVES AT IVOLOAD TRAMsFoRMER NAL HANSFORMKR NORMAL OPERA TING V04. TA 6 E FULL LonaEXCIT'AT/ON In C l/RRENT PERCENT or J' g- FLux .DENSITF-EXC/TATION CURVE/MPROVD TRA NJFORMER Com :11 TIC/VAL TRA NJFORMA'R Alan/v41. UPERA TI/VGPOINT INVENTOR Edgar D. Li lja BY ATTORNEYS W/WWWMMMMW I I I 100 200PRIMARY AM PERE Tun/vs (Rms) Patented Mar. 19, 1940 UNITED STATES PATENTOFFICE ELECTRICAL VOLTAGE TRANSFORMATION APPARATUS Application March 23,1936, Serial No. 70,283

7 Claims.

My invention relates to electrical voltage trans formation apparatus.

Electrical voltage transformation apparatus, embodying my invention, isparticularly useful for supplying low voltage current to the controlcircuits of electrical temperature control units utilized to control oilburners or similar elements of household heating systems, although myinvention is by no means limited thereto but is, on the other hand,susceptible of a wide variety of uses. In such installations, it hasheretofore been the common practice to use an ordinary stepdown statictransformer, as the principal element of the electrical voltagetransformation apparatus, to supply alternating current at a relativelylow voltage of approximately twenty volts; for example, from an ordinaryhousehold lighting circuit normally operating at approximately 110volts.

Particular precautions must be taken in installations of this type inorder to avoid fire hazards and the like resulting from abnormal circuitconditions either in the voltage transformation apparatus itself or inthe secondary circuit to which the apparatus supplies low voltagecurrent. I have found that installations of the type described usingconventional transformers are seriously objectionable in that if thesecondary circuit of the transformer is opened when fullload current orshort-circuit current is flowing therethrough, the interruption oropening of the circuit may result in an arc of comparatively highignition value. A dangerous fire hazard may thus be created if any ofthe conductors in the secondary circuit are broken. In addition, thesecondary voltage may rise to 9. dangerously high value when thesecondary circuit is opened or the primary current may become so largeupon short-circuiting of the secondary circuit that dangerousoverheating of the transformer results. My improved electrical voltagetransformation apparatus is designed to overcome the dangerousconditions resulting from such abnormal circuit conditions.

Two general types of conventional transformers have heretofore beenproposed for use in installations of the type noted. These types oftransformers are commonly designated as low reactance and high reactancetype transformers in. view of the respective inductive reactancecharacteristics thereof. It has been found, however,

that although the high reactance type transformers may not overheat whenthe secondary circuits thereof are short-circuited, severe arcing 55does result upon opening of the secondary circuit when short-circuitcurrent is flowing therethrough and the no-load voltage is frequentlyhigh. On the other hand, a low reactance type transformer will be badlyoverloaded and consequently overheated upon short-circuiting of its 5secondary winding and severe arcing will occur upon opening of thesecondary circuit when short-circuit current is flowing therethrougheven though the no-load voltage of such a transformer is only slightlyhigher than the full-load l0 voltage. My improved voltage transformationapparatus is designed to obviate these objectionable operatingcharacteristics of such high reactance or low reactance typetransformers heretofore used.

It is an object of my invention to provide an improved electricalvoltage transformation apparatus which efficiently supplies current at adesired higher or lower voltage than the voltage of the source of supplyto a secondary circuit and 20 which at the same time effectually limitsthe voltage and current in the secondary circuit to predeterminedsafe-values upon the occurrence of abnormal circuit conditions withineither the secondary circuit or the transformation apparatus itself.

A further object of my invention is to provide an electrical voltagetransformation apparatus including an improved arrangement for limitingthe current flowing in the secondary circuit to a safe value when thesecondary circuit is shortcircuited.

A further object of my invention is to provide an electrical voltagetransformation apparatus including an improved arrangement for limitingthe voltage induced in the secondary circuit to a safe value uponinterruption of the secondary circuit when the secondary circuit isoperating at either full-load or under short-circuit conditions.

A further object of my invention is to provide an electrical voltagetransformation apparatus utilizing a capacitance connected in seriesrelation in the primary circuit of the transformation apparatus andhaving a capacitive reactance substantially equal to the apparentfullload inductive reactance of the primary circuit for increasing theimpedance of the primary circuit when the impedance of the secondarycir- 50 cuit is varied from its full-load value.

Further objects and advantages of my invention will become apparent asthe following description proceeds and the features of novelty whichcharacterize my invention are pointed out with particularity in theclaims annexed to and forming a part of this specification.

For a better understanding of my invention, reference may be had to theaccompanying drawings in which,

Figure 1 is a side elevation of a voltage transformation unit embodyingmy invention, a portion of the front wall of the casing thereof beingbroken away.

Fig. 2 is a side elevation of the transformer included in the apparatusshown in Fig. 1.

Fig. 3 is a sectional view along the line 3-3 of the transformer shownin Fig. 2.

Fig. 4 is a wiring diagram of the apparatus shown in Fig. 1.

Fig. 5 is a wiring diagram of a modified form of voltage transformationapparatus embodying my invention.

Fig. 6 is a wiring diagram of another modified form of voltagetransformation apparatus embodying my invention.

Fig. 7 is a graphic illustration of the no-load line voltage andexcitation characteristics of the voltage transformation apparatus,shown in Fig. l, as compared to the corresponding characteristics of asimilar apparatus provided with a conventional transformer.

Fig. 8 is a graphical representation of the excitation characteristicsof my improved transformer, included in the apparatus shown in Fig. l,as compared to the excitation characteristics of a conventionaltransformer.

.Referring to the drawings, I have shown in Fig. l a voltagetransformation apparatus embodying my invention. In the particular formillustrated, the parts of the apparatus have been designed for use insupplying low voltage current from an ordinary household. lightingsystem to the con trol circuits of electrical temperature regulatingdevices or the like. The parts of the apparatus have been arranged in acompact unitary structure, which is especially rugged in constructionand protects them from damage as well as afiording protection to theuser against accidental contact with any high voltage electricalelements of the apparatus.

In general, electrical voltage transformation apparatus, embodying myinvention, includes as its principal elements a transformer havinginductively coupled primary and secondary circuits and means including acapacitance associated with the primary circuit for minimizing theimpedance of the primary circuit when the secondary circuit is subjectedto full load, and for avoiding any substantial decrease in the impedanceof the primary circuit when the impedance of the secondary circuit isvaried from its full-load value. That is, the Value of the capacitivereactance of the condenser is preferably so chosen with respect to theapparent inductive reactance of the primary winding of the transformerat full-load that these two reactances substantially balance each otherwhen the apparatus is operating at full-load. By the term apparentinductive reactance of the primary, I mean the total inductive effect onthe primary circuit resulting not only from the self-inductance of theprimary winding itself but also from the mutual inductance of thesecondary winding as well, which in turn reflects the efiect of theload. Since the load connected to the secondary winding will, inconjunction with the impedance of the secondary winding itself,determine the current flowing through the secondary winding, this loadwill also afiect the mutual in aieaaes ductive efiect of the secondarywinding and in turn the apparent inductive reactance of the primarywinding. It will thus be seen that the apparent inductive reactance ofthe primary winding varies with changes in load imposed upon thesecondary winding and that consequently a critical value of capacitivereactance is to be used which balances the apparent inductive reactanceof the primary winding at full load.

When a condenser, having a capacitive reactance substantially equal tothe apparent inductive reactance of the primary winding at full load, isconnected in series therewith, the circuit will be tuned substantiallyto resonance at full load. That is, the reactance of the primary circuitwill be substantially zero. As a result, when any change in the load onthe system is made from the normal full-load value, the reactance of theprimary circuit will be increased or at worst decreased only a smallamount. For example, if the secondary winding of the transformer isshort-circuited, the phase relation of the secondary current will bechanged, thus shifting the phase relation of the magnetomotive force ofthe secondary winding, which is in opposition to the magnetomotive forceof the primary winding, and thereby changing the net magnetomotive forceproducing flux in the core and consequently changing the apparentinductive reactance of the primary winding. Then, since the inductivereactance of the primary winding no longer exactly balances thecapacitive reactance of the condenser, the primary circuit will have acomparatively large inductive or capacitive reactance and the impedanceof the primary circuit is little, if any, lower than at full load.Similarly, when the secondary winding of the transformer isopen-circuited, no current flows through the secondary winding and theflux induced by the current flowing through the pri mary winding isunopposed. As a result, the apparent inductive reactance of the primarywinding is increased and exceeds the capacitive reactance of thecondenser so that they no longer balance. In general, then, theimpedance of the primary circuit is at or near its minimum whenfull-load current is supplied from the secondary winding of thetransformer and the impedance of the primary circuit is usuallyincreased, and

at worst decreased very little, thus limiting the current flowingtherethrough, whenever the load imposed on the secondary circuit iseither increased or decreased from its full-load value. The effect of solimiting the current on shortcircuit is to reduce the input power to asmall part of its full-load value.

Referring to the drawings, I have shown in Fig. l a voltagetransformation apparatus embodying my invention. The particularapparatus illustrated has been designed for use in supplying low voltagecurrent from an ordinary household lighting system to the electricalcontrol circuits of temperature regulating devices or the like. Thevoltage transformation apparatus shown in Fig. 1 includes a step-downtransformer ill seecured to the detachable cover ll of a rectangularsheet metal casing 32 by through bolts l3. A transverse sheet metalpartition 54 extends across the interior of the casing 22 and dividesthe same into compartments l5 and 36, respectively. The lower end of thetransformer 50 extends into the compartment it through a suitableaperture formed in the cover ii and the current supply leads H and iiiof the primary winding thereof are thus located within the compartmentl5 of the casing l2. The lead. l1 connects the primary winding of thetransformer III with one terminal l9 of a static condenser 20 which issecured in the bottom of the casing l2 by a strap 2|. The other lead [8of the primary winding of the transformer l0 extends through an aperture22 in the partition l4 and then through an insulating bushing 23surrounding the edges of an aperture in the cover H. The lead l8 thusconnects the primary winding of the transformer II! to one side of asuitable supply line such as a household lighting circuit. The otherside of the supply line is connected to the voltage transformationapparatus by a lead 24 which also extends through the insulating bushing23 and is connected to a terminal 25 of a suitable disconnecting switch26 mounted in the compartment l6 of the casing 12. The switch 26 may beoperated by a manual operating handle or button 21 to open and close thesame and thus connect or disconnect the transformer IO to the supplyline. The other terminal 28 of the switch 26 is connected to a secondterminal 29 of the condenser 20 by a conductor 30 extending through anaperture 3| formed in the partition i4. A layer of insulating materiall4 is preferably positioned on the inner side of the partition 14 toprevent short-circuiting of the condenser terminals i9 and 29 uponaccidental contact thereof with the metal partition 14. It will thus beseenthat the parts of the voltage transformation apparatus shown in Fig.l have been arranged in a compact unitary structure which is especiallyrugged in construction and which protects such parts from damage as wellas affording protection to the user against accidental contact with anyhigh voltage electrical elements of the apparatus.

For reasons set forth hereinafter in greater detail, it is desirablethat the transformer 10 used in my improved voltage transformationapparatus should have a magnetic core structure of such character thatit will not be saturated during any conditions of operation likely to beencountered; that is, the flux produced in the core should bear asubstantially constant ratio to the magnetizing current even during widevariations in the magnetizing current. In order to provide the desiredmagnetic characteristics in the core of the transformer I ll, Ipreferably provide an air gap therein, that is, a non-magnetic section,and I also utilize a core of sufficiently large cross sectional areathat it operates at a comparatively low flux density.

Referring to Figs. 2 and 3, it will be seen that the transformer 10includes a shell-type core 32 provided with a rectangular frame member33 made up of thin substantially rectangular superposed laminations orcore sections of magnetic sheet iron. Rectangular openings 34 are formedin the laminations and a vertical leg or core member 35 extends acrossthe central portion of the openings 34. The leg 35 is made up of thinsuperposed laminations of magnetic iron of substantially the samethickness as the laminations in the frame member 33. The opposite endsof the leg 35 are v-shaped and the lower end 36 thereof is press fittedin a complementary V-shaped notch 31 formed in the frame 33. A similarV-shaped notch 38 is formed in the frame 33 on the opposite side of theopening 34 and receives the V-shaped end 39 of the core leg 35. Thefaces of the end 39 of the core leg 35 are cut off a small amount inorder to provide a clearance v of approximately .002 inch between theupper end 39 of the leg 35 and adjacent surfaces of the notch 38 formedin the laminations 33. Thin shims 40 of brass or other non-magneticmaterial are inserted in the air gap or space between the adjacentsurfaces of the end 39 of the core leg 35 and the notch 38 in order toprevent longitudinal displacement of the core leg 35.

The core leg 35 and shims 40 are press fitted into position within thenotches 31 and 38 formed in the laminations 33 and the through belts I3are inserted in holes formed-.,by complementary half-round grooves 4|and 42 formed at the base of the notches 31 and 38 and in the ends 36and 39 of the core leg 35, respectively, to hold the parts of themagnetic core 32 rigidly in position. These bolts l3 also pass throughregistering holes formed in. a cup-shaped sheet metal end shield 43arranged on the upper side of the transformer and serve to hold the samein position thereon. It will be noted that the lateral edges of the endshield 43 register with the adjacent edges of the laminations 33.

The transformer I0 is provided with inductively coupled primary andsecondary windings 44 and 45 which are arranged concentrically about thecore leg 35 and mounted thereon. A layer of insulation 46 is interposedbetween the adjacent inner surfaces of the secondary winding 45 and thelateral surfaces of the core leg 35. Also a layer of paper 41 or similarinsulating material is interposed between the adjacent superposedsurfaces of the primary and secondary windings 44 and 45. The secondarywinding 45 is provided with a relatively smallnumber of turns ascompared with the primary winding 44 so that when a relatively largevoltage is impressed on the primary winding 44, a relatively smallvoltage will be induced in the secondary winding 45.

The primary winding 44 is provided with leads l1 and 18 through whichelectrical energy is supplied thereto from a suitable source ofalternating current such as a domestic lighting system. As is shown inthe wiring diagram in Fig. 4, the condenser 20 is connected in seriesrelation with the primary winding of the transformer Ill. The value ofthe capacitive reactance of the condenser 29 is so designed during themanufacture of the apparatus that it is substantially equal to theapparent inductive reactance of the primary winding 44 of thetransformer l0 when full-load current is flowing through the second-'ary winding 45 thereof. In other words, the impedance characteristics ofthe transformer and condenser are so related that a condition of seriesresonance will be had at full-load, as was described above. In theparticular voltage transformation apparatus illustrated, which isadapted to supply approximately 50 volt amperes at 25 volts and at alagging power factor of about 55 percent from a volt 60 cycle supplycircuit, I have found that satisfactory operation is had if a condenserof approximately 5 microfarads tion and disconnection of the load fromthe secondary winding I have found that if a condenser is connected inseries relation with the primary winding of a'conventional transformerin such manner that the primary circuit is in series resonance when theapparatus is operating under full load, undesirable or abnormal voltageand current conditions are likely to occur during certain conditions ofoperation of the apparatus. Because these abnormal voltage and currentconditions arise from the effects of certain transient electricalphenomena, their existence would not be readily apparent from atheoretical analysis of the steady state conditions prevailing in suchan apparatus. This abnormal operation may best be understood byconsidering the operation of an apparatus diagrammatically illustratedin'Fig. 4 when a conventional transformer is substituted for my improvedtransformer in therein. Such a conventional transformer might be similarto the transformer ill except that no air gap would be provided thereinand the cross-sectional area of the core would be relatively smaller sothat the flux density therein would be relatively high.

The excitation characteristics of such a conventional transformer areindicated by curve 55 in Fig. 8, which shows the relation of the peakflux density in kilolines per square inch to the primary ampere-turns ofthe transformer. The normal operating point for such a transformer isindicated at 55 on the curve 55 showing that the flux density isapproximately 70 kilolines per square inch and comparatively close tothe saturation point of the transformer core. It should be noted thatthe bending of the curve 55, as the primary ampere turns are increased,indicates that after a flux density of approximately kilolines persquare inch is obtained, comparatively little increase in flux densityis had even though the magnetizing current or primary ampere turns aregreatly increased.

As the capacitive reactance of the condenser, to be connected in seriesrelation with the pri mary winding of the hypothetical conventionaltransformer, is designed to equal the apparent inductive reactance ofthe primary winding at full load, the primary circuit will besubstantially in series resonance at full load. Consequently, wheneverthe loadcurrent is increased or de= creased, the reactance of theprimary circuit will be increased, thus limiting the current flowingtherethrough. I have found, however, that if the secondary circuit ofthe conventional transformer is open circuited when full-load orshortcircuit current is flowing therethrough, the primary current andsecondary voltage of the conventional transformer may rise todangerously high values, these values being sustained so long as theprimary circuit remains energized.

This action of the conventional transformer upon opening of itssecondary circuit under load may best be understood by referring to Fig.7 as well as Fig. 8 noted above. Curve 5? in Fig. 7 shows the relationof the linevoltage to the excitation current in percent of full-loadcurrent when the apparatus is operating at no-load, that is, with thesecondary circuit open. From an inspection of curve 5?, it will be seenthat the circuit exhibits negative impedance characteristics from points58 to 59. Between these points the line voltage required to maintain agiven current decreases as the current increases. Beyond point 55}, anincrease in voltage is again required to increase the current Thisnegative greases impedance characteristic is probably due to the factthat the impedance of the conventional. transformer decreases frompoints 5% to 59 upon an increase of current flowing therethrough. Thedecrease in impedance of the transformer results from magneticsaturation of the transformer core, for upon referring to Fig. 8 it willbe seen that when the excitation current is in creased above the normaloperating point the flux induced in the transformer core increases acomparatively small amount. Thus, since the reactance of the primarywinding of the transformer depends upon the amount of flux induced inthe transformer core, the reactance of the transformer will decreaseafter saturation of the core has been reached. The saturation of thetransformer core is accompanied by a distortion of the wave form of thealternating current flowing through the primary circuit so that high frequency harmonics are introduced into the current. The capacitivereactance of the condenser varies inversely as the frequency of thecurrent flowing therethrough and consequently offers little impedance tothe high harmonic currents resulting from the distortion in wave form ofthe primary current. It is believed that this decreased reactance of thecondenser is responsible for prolonging the negative impedance effectinitiated by the conventional transformer. Beyond point 59 on curve 57,the impedance of the primary circuit again increases because thesubstantially non-distorting resistance and leakage reactance of thetransformer form a greater part of the total impedance thereof andpredominate over the effect of the core-flux reactance in determiningthe transformer impedance and exciting current-wave form.

It will thus be seen that a voltage transformation apparatus embodying atransformer and a condenser connected in series with the primary thereofhas a negative impedance characteristic of such form that there is acomparatively large range of line voltage over which the no-load primarycurrent may have more than one value;

for example, a conventional transformer in. such an apparatus might,under no-load conditions, operate at either points 60 or 6| on curve 61.If current is first supplied to the primary winding of the conventionaltransformer with the secondary winding open-circuited, operation wouldordinarily take place at point 60 and the primary winding would draw ano-load exciting current corresponding to the value indicated at point6% on curve 51. On the other hand, if the conventional transformer isoperated at full load and the secondary circuit open-circuited duringsuch time, the primary winding may in the subsequent no-load operationdraw an exciting current corresponding to the value indicated at eitherpoint 60 or point 6| indicated on curve 57. If the alternating excitingcurrent happened to be passing through the zero point at the instant theload was removed from the secondary circuit, no-load operation wouldprobably be had with a primary exciting current corresponding to the ivalue indicated at point 60, which may conveniently be termed the normalno-load exciting current. If, however, the instantaneous primary currentat the time of interruption of the sec ondary circuit is comparativelylarge, no-load 70 operation would probably occur with an abnormalprimary exciting current of a value corresponding to point (H on curve51. This result probably obtains because the magnetomotive forceproduced by the load component of the primary current is momentarilyunopposed by the magnetomotive force produced by the secondary current,the latter having been cut off upon open-circuiting the secondarywinding The unopposed flux thus produced by the load component of theprimary current saturates the transformer core and establishes no-loadoperation at the abnormally high no-load exciting current valueindicated by point 6| on curve 51. Consequently, a high secondaryvoltage would appear. and severe arcing would take place during theopening of the switch 54. It will be seen that the value of the excitingcurrent at point 6| on curve 51 is more than five times the value of thefull-load current and the transformer would, therefore, soon beoverheated. In addition, a dangerous over-voltage would be imposed onthe condenser connected in the primary circuit since the circuit is ator near resonance and the current is abnormally high so the voltageacross the condenser may be several times as great as the line voltage.Moreover, the high secondary voltage appearing at the terminals of thesecondary winding of the transformer, because of the abnormally highprimary exciting current, may result in a hazardous condition.

The improved transformer, which I have illustrated in connection withthe preferred embodiment of my invention described above, isparticularly effective in obviating the abnormal voltage and currentcondition encountered with the conventional transformer described above.Although it embodies other improvements in design and construction, thetwo fundamental characteristics of my transformer, which are especiallyeffective in obviating the defects of operation described, are that myimproved transformer normally operates with a comparatively low fluxdensity in its core and draws a substantially unf distorted magnetizingcurrent because of the air gap in the core.

In general, the low flux density of the core is the most importantfactor and if it is had, im proved operation will result even though theair gap is omitted. The excitation characteristics of curve 55 for aconventional transformer.

my improved transformer are indicated by curve 53 in Fig. 8. The greaterreluctance of the ma netic core used in my transformer causes the curve63 to fallsomewhat below the excitation That is, a greater excitation isrequired to produce a flux of the same density in the core. Also, thecomparatively large cross-sectional area of the magnetic core used in myimproved transformer makes it possible normally to operate the sameunder full load at the comparatively low flux density of approximately30 kilolines per square inch, as indicated at point 54 on curve 63. Thealtered magnetic characteristics of my improved transformer as comparedwith those of a conventional transformer improve its operation at noload and at normal full load, and-also prevent the abnormal conditionsreferred to above in connection with the operation of a conventionaltransformer.

The improvement in operation effected by the change in magneticcharacteristics in my improved transformer at normal full load resultsfrom the fact that the transformer is operated at a lower flux densityand therefore, with lower core losses than a conventional transformer ofthe same general size and rating. In addition, the copper losses in myimproved transformer are lower since the number of turns in the windingsis decreased. The no-load losses in my improved transformer are lowbecause at no load, the voltage impressed on the transformer isconsiderably below that at full load and the flux density iscorrespondingly reduced below the normal full-load value of 30 kilolinesper square inch.

The most important effect of the change in magnetic characteristicsutilized in my improved transformer is in preventing the conditions ofabnormal operation referred to above with respect to the operation of aconventional transformer. This improved operation may best be understoodby reference to Fig. '7 in which the relation between line voltage andexcitation current expressed in percent of full-load current isillustrated by curve 65. Upon reference to this curve it will be seenthat my'improved transformer also displays a negative impedancecharacteristic. For example, between points 66 and 61 on curve 65 theexcitation current increases as the line voltage is decreased. There isone important difference, however, in that the low point 61 is now of avalue greater than the normal operating or line voltage applied to thetransformer. In fact, upon reference to Fig. '7 it will be seen that thepoint 61 represents a very much higher voltage than the correspondingpoint 59 on curve 51 for the conventional transformer. This differencein operating characteristics probably results primarily from the factthat the lesser density of flux in the transformer core reduces the dropin transformer impedance. It also results in less current wavedistortion and consequently minimizes the change in capacitive reactanceof the condenser. Furthermore, when the secondary circuit is openedunder full load, for example, the transient counter electromotive forceor voltage induced in the primary winding of the transformer during theopening of the secondary circuit is higher and more effective inreducing the primary current from its fullload value to its normalno-load value indicated at point 62, in view of the fact that thetransformer core does not become saturated even when the primary currentis greatly increased. The subnormally saturated transformer core, whichI have provided, thus constitutes a means utilizing a transient counterelectromotive force generated in the primary winding upon disconnectionof the load from the secondary winding to prevent the sustained flow ofan abnormal primary exciting current of substantially greater value thanthe normal no-load primary exciting current.

My improved voltage transformation apparatus is also especiallyeffective in limiting the primary current to a safe value when thesecondary circuit is either accidentally or deliberatelyshort-circuited. Thus if the secondary winding 45 of the transformer I0is short-circuited, the phase relation of the secondary current will bechanged and the apparent inductive reactance of the primary winding willbe correspondingly changed. As a result, the capacitive reactance of thecondenser 20 is no longer balanced by the inductive reactance of theprimary winding 44 of the transformer so that the total impedance of theprimary circuit is increased, thus limiting the primary'current to acomparatively low value when the secondary circuit is short-circuited.Consequently, if a short circuit occurs in the secondary circuit of thevoltage transformation apparatus described above the power supplied tothe apparatus is decreased,

or similar protective devices.

through leads 172 and 73.

It is also unnecessary to provide any fuses or other protective devicesfor my improved voltage transformation apparatus to guard againstpossible harmful results, caused by a failure of any of the elements ofthe primary circuit. That is, if either the primary winding fi l of thetransformer ll] of the condenser are open-circuited, no current williiow through the primary circuit and the apparatus will simply becomeinoperative. If, on the other hand, the primary Winding dd of thetransformer it is short circuited the capacitive reactance of thecondenser will no longer be balanced by the apparent inductive reactanceof the primary winding of the transformer and the resulting increase intotal impedance of the primary circuit will limit the current flowingtherein to a comparatively small value. Conversely, if the condenser 29is short-circuited the apparent inductive reactance of the primarywinding dd will no longer be balanced by the capacitive reactance of thecondenser 2D and as a result, the current flowing in the primary circuitwill be limited to a comparatively small value, due to the increase intotal impedance of the primary circuit.

Moreover, my improved voltage transformation apparatus improves thepower factor of the circuit to which it is connected. This improvementin power factor is brought about by the leading current drawn by thecondenser 20 and in this respect, my improved voltage transformationapparatus is particularly desirable as compared to the conventionaltransformer supply arrangement, which draws a lagging current. Theimprovement in power factor is especially important when several voltagetransformation units are used to supply a variety of control circuits inany particular installation.

In Fig. 5 I have illustrated the wiring diagram of a modified form ofvoltage transformation apparatus embodying my invention. The arrangementshown in r'g. 5 includes a conventional step down transformer providedwith a primary winding and a secondary winding 710, inductively coupledthrough a magnetic core 7]. The primary winding is provided with arelatively large number of turns as compared to the secondary windingid, in order that the secondary voltage induced in the winding "1U willbe relatively small as compared to the voltage impressed on the primarywinding Elem trical energy is supplied to the primary winding tilthrough a suitable source of alternating current, such as a householdlighting circuit, A suitable manually operable switch id is inserted inthe lead in order to facilitate connection and disconnection of thevoltage transformation apparatus from the supply circuit.

A condenser id is connected to the lead 13 in series relation with theprimary winding lid of the transformer 68. The capacity of the condenser15 is so chosen that the capacitive reactance of the condenser issubstantially equal to the apparent inductive reactance of the primarywinding 69 when the apparatus is operating under full, load.Consequently. the impedance of the primary circuit is near its minimumat full load and is decreased little, if any, whenever the load on thesecondary circuit is varied from its full-load value as was describedabove with respect to the apparatus shown in Figs. 1-4, inclusive.

The secondary winding it is connected to a suitable load through leadsl6 and Ti. In Fig. 5

arouses this load is diagrammatically illustrated as an electric motorprovided with 2. held winding "13 and a rotor A manually operable switchtil is inserted in the lead 'l'l to facilitate connection anddisconnection of the load from the secondary winding it.

As was pointed out in detail above, if a conventional transformer isutilized with a condenser connected in series relation with the primarywinding thereof, the primary circuit being adjusted for series resonanceat full load, dificulty may be encountered in the operation of theapparatus when the secondary circuit is interrupted if full-load or.short-circuit current is flowing therethrough. For the reasons pointedout above, the primary current as well as the secondary voltage may riseto dangerously high values under such circumstances. In order to avoidsuch abnormal operation I have provided a resistance, which isassociated with the primary cir uit.

In the arrangement shown in Fig. 5 a resistance 3] is preferablyconnected in series relation with the condenser 75 and primary windingof the transformer :63. troduces an additional voltage drop in theprimary circuit and thereby lowers the voltage impressed across thecondenser 15 and primary winding respectively. Since the voltage dropacross the resistance SJ is the product of its resistance and thecurrent flowing therethroug this voltage drop increases proportionatelyas the primary current increases. That is, the voltage impressed on theprimary winding '39 and condenser is progressively decreased as theprimary current increases. Turning now to curve in Fig. l, whichrepresents the relation of line voltage to excitation current for aconventional transformer having a condenser connected in series with theprimary thereof, it will be seen i that the effect of the resistance :Siis to move the point on the curve 57 upward and to the left. That is,the steadily increasing voltage drop across the resistance 8] withincreasing primary current tends to counterbalance the negative impedane characteristic of the primary cire cult shown in curve between thepoints 53 and Consequently, the resistance may be given a sirfficientlylarge value so that the point is raised to a value greater than thenormal operating voltage applied to the primary circuit. Consequently,abnormal nc-load operation will he entirely prevented when such a valueof resistance is used. In other words, when such a resistance isinserted in the primary circuit, the

no-load primary exciting current will be limited to a valuesubstantially equal to that indicated at point ti) on curve 57, evenwhen the secondary circuit is open-circuited either under load, or undershort-circuit conditions.

I have shown in Fig. 6 a second modified form of voltage transformationapparatus embodying my invention and utilizing a conventionaltransformer. The arrangement shown in Fig. 6 is very similar to thatshown in Fig. 5 with the oneimportant difference that a resistance iscon nected in parallel with the condenser in the primary circuit ratherthan in series therewith. The same numerals have been used to indicateidentical parts in Fig. 6 corresponding with those shown in Fig. 5.Thusa conventional step-down transformer 68 is provided, having primaryand secondary windings B9 and lid, respectively, inductively coupledthrough a magnetic core H. Electrical energy is supplied to the primarywind- The resistance in- 1 N A condenser 15 is connected in seriesrelation with the primary winding 69 of the transformer 68. As wasdescribed above with respect to the apparatus shown in Figs. 1-4,inclusive, as well as Fig. 5, the capacity of the condenser is so chosenwith respect to the apparent inductive reactance of the primary windingthat the primary circuit is in series resonance at full load.

In order to avoid abnormal voltage and current conditions uponinterruption of the secondary circuit when the apparatus is eithershortcircuited on the secondary side or is operating under full load, aresistance 82 is connected in parallel relation with the condenser 75and thus shunts the same. The effect of the resistance 82 shown in Fig.6 is very similar to the effect of the resistance 8| shown in Fig. 5;that is, the circuit is less sharply resonant. The resistance r 82 ismade small enough to decrease the negative impedance characteristic ofthe primary circuit to such an extent that the point 59 on the curve 51in Fig. 7 is moved upwardly and to the left until it attains a valuegreater than the normal operating voltage for the circuit. As a result,abnormal no-load operation is entirely prevented so that when thesecondary circuit is open-circuited, the primary exciting current willbe limited to its normal small value indicated by the point 60 on curve51 in Fig, 7 even though full-load or short-circuit current is flowingthrough the secondary circuit at the instant of interruption.

The arrangements shown in Figs. and 6 are each effective in limiting theprimary current to a safe value when the secondary circuit isshort-circuited. If the secondary circuit is shortcircuited for anyreason the change in phase relation of the current flowing therein andthereby changes the apparent inductive reactance of the transformerprimary winding. As a result, the capacitive reactanceof the condenseris no longer balanced by the apparent inductive reactance of thetransformer primary. The impedance of the primary circuit isconsequently increased and the primary current is thus limited to a safevalue. It will be seen, however, that the arrangements shown in Figs. 5and 6 are not so effective as that shown in Figs. 1 to 4, inclusive, inlimiting the primary current in case of a short circuit in the secondarybecause the resistances inserted in the primary circuit make up aportion of the total impedance thereof. This portion of the primarycircuit is unaffected by changes in the inductive or capacitivereactance of the circuit so that the tuning of the primary circuit isless sharp and the impedance shows less relative change with changes inthe inductive or capacitive reactance of the circuit. The currents areconsequently greater, upon short-circuiting the secondary circuit, inthe apparatus shown in Figs. 5 and 6 than in that illustrated in Figs. 1to 4, inclusive.

It will be understood that the parts of the voltage transformationapparatus shown either in Fig. 5 or Fig. 6 may be mounted in a compactunitary structure or a unit substantially like that shown in Fig. 1 ifso desired.

Although I have shown certain specific embodiments of my invention whichare particularly adapted for use in supplying energy to temperaturecontrol units for household heating systems, it will be understood thatI do not desire to limit my invention to the particular constructionshown and described, but intend, in the appended claims, to cover allmodifications within the spirit and scope of my invention.

I claim as my invention:

1. An electrical; voltage transformation apparatus comprising, incombination, a primary circuit, means for connecting said primary cir-.

cuit to a source of alternating current, a secondary circuit, meansincluding a magnetic core associated with said primary and secondarycir-. ,cuits for inductively coupling the same and for minimizing theopen-circuit voltage induced in said secondary circuit, said magneticcore having ample cross-sectional area to operate at relatively low fluxdensity when all load values of current from zero to full load areflowing through said secondary circuit, and means including a capacityassociated with said primary circuit for minimizing the impedancethereof when said secondary circuit is operating at full-load and forautomatically preventing a substantial decrease in the impedance of saidprimary circuit when the impedance of said secondary circuit is variedfrom its full-load value.

2. An electrical voltage transformation apparatus comprising, incombination, inductively coupled primary and secondary windings, meansfor connecting said primary winding to a source of alternating current,means for supplying alternating current from said secondary winding to aload, and a condenser connected in series relation with said primarywinding, said condenser having a capacitive reactance substantiallyequal to the apparent inductive reactance of said primary winding .whenfull-load current is supplied from said secondary winding to the load,and means for limiting the open-circuit voltage induced in saidsecondary winding to a value substantially less than the full-load valueof said voltage.

3. An electrical voltage transformation apparatus comprising, incombination, a transformer provided with inductively coupled primary andsecondary windings, means including a condenser associated with saidprimary winding for minimizing the short-circuit voltage induced in saidsecondary winding, and means for minimizing the open-circuit voltageinduced in said secondary winding.

4. An electrical voltage transformation apparatus comprising, incombination, juxtaposed primary and secondary windings, means forconnecting said primary winding to a source of alternating current,means for supplying current from said secondary winding to a load, meansincluding a magnetic core extending axially through said windings andforming a substantially continuous magnetic path about the same forinductively coupling said windings, means including a narrownon-magnetic section formed means including a condenser connected. inseries relation with said primary winding for minimizing the impedancethereof when said secondary circuit is operating at fu1l-load and forautomatically preventing a substantial decrease in the impedance ofsaid. primary circuitwhen the impedance of said secondary circuit isvaried from its full-load value, said condenser having a capacitivereactance substantially equal to the apparent inductive reactance ofsaid primary winding when full-load current is supplied from saidsecondary winding to the load.

,5. An electrical voltage transformation ap paratus comprising, incombination, inductively coupled primary and secondary windings, meansfor connecting said primary winding to a source of alternating current,means for supplying current from said secondary winding to a load, acondenser connected in series relation with said primary winding, saidcondenser having a capacitive reactance substantially equal to theapparent inductive reactance of said primary winding when full-loadcurrent is supplied from said secondary winding to the load, and meansutilizing a transient counter electromotive force generated in saidprimary winding upon disconnection of the load from said secondarywinding for preventing the sustained flow of a primary exciting currentof substantially greater value than the normal no-load primary excitingcurrent.

6. An electrical voltage transformation apparatus comprising, incombination, a magnetic core, inductively coupled primary and secondarywindings mounted on said core, means for conmeeting said primary windingto a source of alternating current, means for supplying current fromsaid secondary winding to a load, a condenser connected in seriesrelation with said primary winding; said condenser having a capacitivereactance substantially equal to the apparent inductive reactance ofsaid. primary winding when full-load current is supplied from saidsecondary winding to the load, and means including a substantiallynon-inductive resistance associated with said primary winding forminimizing the no-load exciting current of said primary wLnding.

7. An electrical voltage transformation apparatus comprising, incombination, a primary winding, means for connecting said primarywinding to a source of alternating current, a secondary winding,magnetic means coupling said primary and secondary windings, saidmagnetic coupling means comprising a metallic core having across-sectional area to provide a relatively low flux density in saidcore for all conditions of operation from zero to full load, and acondenser connected in series relation with said primary winding havinga capacitive reactance substantially equal to the apparent inductivereactance of said primary winding when full load current is supplied tothe load by said secondary winding.

EDGAR. D. LILJA.

